A construction machine, such as an excavator and a wheel loader, generally includes a hydraulic pump driven by an engine, and a hydraulic system for driving a plurality or working machines, such as a boom, an arm, a bucket, a travel motor, and a turning motor, through pressure of working oil discharged from the hydraulic pump.
The hydraulic pump used in the hydraulic system of the construction machine is a variable capacity type pump including a swash plate formed inside the pump and an adjusting device for adjusting an angle of the swash plate (swash plate angle), and especially, may be divided into a machine control type or an electronic control type according to a type of an instruction input in the adjusting device in order to adjust the swash plate angle.
The initial hydraulic pump mainly employs the machine control method, but the electronic control type for controlling the swash plate angle by applying an electric signal to the adjusting device has been introduced today. The hydraulic pump of the electronic control type includes a so-called pressure control type electric hydraulic pump.
The pressure control type electric hydraulic pump is controlled by a control means, such as an electronic control unit.
The electronic control unit receives a value of a pressure sensor according to an operation of a lever, such as a joystick, within an operation seat of a construction machine, and a value of a swash plate angle from a sensor mounted inside the electric hydraulic pump as electric signals, respectively, and outputs the electric signal for controlling pressure to the corresponding electric hydraulic pump.
For example, the electronic control unit includes an input unit for receiving the values detected from the sensors, a calculation unit for generating a corresponding control signal based on the input value, and an output unit for outputting the control signal to the electric hydraulic pump.
In a case of a construction machine using the electric hydraulic pump, when the electronic control unit fails to operate, for example, when any one of the input unit receiving the electric signal and the output unit outputting the control signal has failure, the electric hydraulic pump may not be normally controlled, thereby causing an even worst risk, such as failure of driving the construction machine itself using the electric hydraulic pump.
Accordingly, a method of handling an emergency situation, such as operation failure of the electronic control unit is prepared by providing an emergency control unit so as to temporarily control the electric hydraulic pump when the electronic control unit fails to operate.
FIG. 1 is a hydraulic circuit diagram illustrating an example of a hydraulic system using an electric hydraulic pump in the related art.
Referring to FIG. 1, a construction machine includes first and second electric hydraulic pumps 10a and 10b driven by an engine, a plurality of main control valves 20a, 20b, 20c, and 20d for controlling a flow of working oil discharged from the electric hydraulic pump, first and second travel motors 30a and 30b which may be driven with the working oil supplied from the main control valves, and a plurality of working machines 40a and 40b. 
Further, the construction machine includes a predetermined hydraulic line forming a path, through which the working oil is transferred, by connecting the pumps, the main control valves, the working machines, and the like, and further includes a straight travel control valve 70 capable of changing a supply path of the working oil for the travel motors 30a and 30b and the working machines 40a and 40b on the hydraulic line between the pumps and the main control valves.
Further, the construction machine includes adjusting devices 12a and 12b for adjusting a swash plate angle of the first and second electric hydraulic pumps 10a and 10b, and an electronic control unit 50 capable of controlling the adjusting devices, and the electronic control unit 50 receives a pressure signal 80 of a joystick (not illustrated) and flow signals (for example, angle detection signal of the swash plate angle) 14a and 14b of the respective pumps 10a and 10b and generates corresponding control signals 52a, 52b, and 54, and outputs the control signals to the adjusting devices 12a and 12b and the straight travel control valve 70 of each pump.
Further, the construction machine further includes an emergency control unit 60 for preparing operation failure of the electronic control unit in the hydraulic system.
FIG. 2 is a logic circuit diagram illustrating an example of the emergency control unit 60 of FIG. 1. Referring to FIG. 2, when the electronic control unit (reference numeral 50 of FIG. 1) fails to operate, the emergency control unit 60 may switch, for example, a path of a control signal transmitted from input ports 62A and 62B to output ports 62a and 62b to a substitute path through which the control signal is transmitted from a regular power source 64, such as a battery, to the output ports 62a and 62b through an operation of a switch SW.
That is, a path with a solid line (the control signal transmitted from the input ports) may be switched to a path with a dotted line (the control signal transmitted from the regular power source) based on the switch of FIG. 2.
In this case, the control signal of the regular power source 64 transmitted to the output ports 62a and 62b may be determined as a predetermined value through resistors R1 and R2 arranged on the path.
The emergency control unit including the aforementioned configuration in the related art is configured so that the respective electric hydraulic pumps 10a and 10b maintain the predetermined same flow, and thus the construction machine may perform load work with pressure equal to or lower than predetermined pressure at an emergency situation where the electronic control unit fails to operate.
That is, regardless of the electronic control unit, the electric hydraulic pump may be temporarily controlled so that the construction machine may minimally perform work or travel.
FIG. 3 is a graph illustrating a relationship between pressure and the quantity of flow when the electric hydraulic pump is driven according to an operation of the emergency control unit. As illustrated in FIG. 3, in the hydraulic system in the related art, when it is assumed that the maximum quantity of outlet flow of one electric hydraulic pump at the RPM of a rated load of the engine is Qmax, the quantity of outlet flow of the hydraulic system in the related art is fixed as a maximum value of 2×Qmax according to the operation of the emergency control unit, and load work corresponding to predetermined pressure (for example, P1) may be performed at the maximum quantity of flow.
The load work corresponding to P1 may generally be low load work, such as minimum driving or travel of the working machine.
However, when load work (for example, high load work) corresponding to higher pressure than the predetermined pressure P1 is desired to be performed, load equal to or larger than power of the engine is applied to the pump, so that the engine is stalled, thereby resulting in the worst situation where driving itself of the construction machine is impossible.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.